In recent years, there have been proposed and developed various combined resonators, each of which is configured to provide different resonance frequencies. One such combined resonator has been disclosed in Japanese Patent Provisional Publication No. 8-338330 (hereinafter is referred to as “JP8-338330”). In the combined-resonator equipped air intake duct disclosed in JP8-338330, a plurality of resonators are combined and connected in series to each other. The combined resonator can provide the same number of resonance frequencies as the number of the resonators connected to each other. In more detail, in the combined-resonator equipped air intake duct disclosed in JP8-338330, a first one of the plurality of resonators is communicated with the air intake duct through a first insertion conduit, whereas the second resonator is communicated with the first resonator through a second insertion conduit.